Do I have this right? The manifold has a taper to the inside,looking like the pipe would have to be the opposite of the bell shape that most GM's use? If this is what you have,there's a double beveled packing that fits into the manifold bell,and has the same shape on the muffler side,so the correct sized bell shaped flange would slide down on the pipe and attach to the manifold with either 2 or 3 bolts, compress the packing around the pipe,and squeeze the packing into the bell part of the manifold,making a tight seal. You should be able to get these packings in a variety of sizes and materials with different hardness,so they will be soft enough to compress but hard enough to last a while. If I misunderstood your design,never mind...
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